Where are you getting your prices and part numbers?
Checking gmpartsgiant.com and autozone.com , both sites indicate the same starter is used for both motors , 3.5L/LZ4 and 3.9L/LZ9 . So there should be no difference in price.
Autozone lists an AC-Delco reman for about 190 dollars, p/n 336-2140A .
HTH.
Doug
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